Sound
The notion of sound is rather remarkable. Something happens there and we know it here, even if we are looking the other way, not paying attention, or even asleep. The fact that some sounds can produce physical and emotional effects is just short of astounding. These notes will perhaps remove some of the mystery associated with sound and hearing, but probably none of the wonder.
Sound is...
Sound is a disturbance of the atmosphere that human beings can hear. Such disturbances are produced by practically everything that moves, especially if it moves
quickly or in a rapid and repetitive manner.
Sound moves
You should be aware that the air is made up of molecules. Most of the characteristics we expect of air are a result of the fact that these particular molecules are very
light and are in extremely rapid but disorganized motion. This motion spreads the molecules out evenly, so that any part of an enclosed space has just as many molecules as any other. If a little extra volume were to be suddenly added to the enclosed space (say by moving a piston into a box), the molecules nearest the new volume would move into the recently created void, and all the others would move a little farther apart to keep the distribution even.
Because the motion of the molecules is so disorganized, this filling of the void takes more time than you might think, and the redistribution of the rest of the air molecules in the room takes even longer. If the room were ten feet across, the whole process might take 1/100 of a second or so.
If the piston were to move out suddenly, the volume of the room would be reduced and the reverse process would take place, again taking a hundredth of a second until everything was settled down. No matter how far or how quickly the piston is moved, it always takes the same time for the molecules to even out.
In other words, the disturbance caused by the piston moves at a constant rate through the air. If you could make the disturbance visible somehow, you would see it spreading spherically from the piston, like an expanding balloon. Because the process is so similar to what happens when you drop an apple into a bucket, we call the disturbance line the wavefront.
If the piston were to move in and out repetitively at a rate between 20 and 20,000 times a second, a series of evely spaced wavefronts would be produced, and we would hear a steady tone. (One wavefront is heard as a click.) The distance between wavefronts is called wavelength.
The Physics of Sound
Sound in air is the transfer of periodic movements between adjacent colliding atoms or molecules. This sonic energy typically expands away from the site of the collisions as a spherical or bubble-shaped emanation, the surface of which is in a state of radial oscillation.
The sonic bubble expands and contracts with the same periodicities as the initiating sound source. The accepted model of sound waves is incomplete because it uses the graphical representation of the mathematical law of sinusoidal energy, typically given as amplitude in the vertical axis versus time in the horizontal. While this is correct in terms of graphical depiction, it is not how the energy actually moves through space.
Sound in air does not travel as longitudinal waves as is commonly described in physics text books. Sound propagates spherically in air due to diffraction, the reactive result of atomic collisions. Reciprocal effects in air occur in the jostling of molecules initiated by a sound event, causing components of the sonic energy to move in all directions almost simultaneously. The distribution of energy within the sonic bubble is always concentrated on axis with the direction of primary propagation from the sound source.
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